Radio Servicing Information

For use by legally qualified and licensed repairers only

Countries have laws regulating who is legally allowed to repair or work on electrical equipment that will be plugged into the mains electricity supply. This is to protect the public from the risk of electric shock. If you are not lawfully allowed to repair electrical equipment yourself, you must find an electrician, a licensed electrical mechanic or electrical contractor or electronic technician to do any work on the equipment.

Different countries use electricity supply at different voltages. The United States uses 110-120 volts AC. The United Kingdom and Australia (amongst many other countries) uses 240 volts AC. Any electric shock at Voltage 110v or above is likely to be lethal.

Many types of early consumer electronic equipment was constructed at a time when electrical safety was not as carefully considered as is required under manufacturing laws today, and many early radio sets etc have mains electricity wiring and metal mains voltage terminals fully exposed to be touched once the metal chassis is removed from the wooden case or bakelite housing.

This is why any electrical work to restore or repair an old radio must be done by a licensed qualified technician.

ALIGNING RADIOSThe tuned circuits in any electronic equipment are designed to resonate at a particular frequency and can often have changed, due to age, component failure, or miss-tuning. If the radio is working satisfactorily, then leave well enough alone. Tuning attempts can often degrade performance.

I will be describing valve radios, and using older terms like kilocycles (kcs) megacycles (mcs), Automatic Volume Control (AVC) and plate and grid. The alignment techniques also apply to modern solid state electronics, except the voltages are lower and the impedances are not as high.

Before starting, identify which tuned circuits need aligning, which are best left alone, and where they are actually located. Identify any pitfalls, for example, hard to get at screws, high voltage points nearby, possible capacity effects, complicated tuning requirements, and fragile components. Freeing the tuning screws can be done before hand if you want to, but first mark the position of any slot with a small pencil line. Tuned circuits are aligned by changing the capacitance or inductance. The capacity can be changed by moving two pieces of metal closer together. A compression trimmer, beehive trimmer, piston trimmer, or rod trimmer are some common types. The inductance can be changed by squeezing the coil turns closer together, or moving some material into the coil. The material may be a dark coloured round lump of iron material, called a "ferrite" or "slug" .

Get your tools ready. Buy a set of nylon alignment tools, which have brass slot end for screws, and a hex drive for beehive trimmers. If there is a trimming capacitor inside a shield, then you may need a thin screwdriver, but insulate the shaft with tape or heatshrink as it may short out the HT (High Tension) while tuning. The tuning device is commonly a slotted head brass screw or ferrite slug,but it can also be a hexagonal slug, clamped rod, or the bending of some metal. Some AWA sets have a vertical rod that is pushed in or out of a trimming capacitor, and it is clamped by a hex nut. Make up a hex tube spanner from a piece of straight copper tube beforehand. File the hex shape to fit. Some slugs have a spring or nut to hold them in place, but most will be fixed by some sort of glue, wax, or epoxy resin. The epoxy ones are impossible to move and you will break the slugs by trying, so do not. The wax can be melted or scraped away. The glue can be broken. Using a screwdriver in a slot head slug can break it, so only use a soft tool like the nylon type.

Get your test equipment ready. A signal generator covering 50 khz to 30 mhz, is useful for domestic radios and shortwave receivers. This is usually all that is required. You can add more sophisticated equipment, like a sweep generator and oscilloscope which is useful for bandpass filters in communications receivers. You can add a frequency counter for accurate frequency setting. You can add a level meter for output level measurement, as this will get you more accuracy, than tuning than by ear.

PROBLEMSIf you break a slug, glue it back together, as the total ferrite mass is all than is needed. It does not have to be continuous. Use the minimum of glue. If it screws into a close fitting coil, make sure it is straight. If you shatter a slug, get a new one. If the brass rod breaks off, replace it, file a new slot in the end, solder a nut on the top, any neat tidy solution will do. If a slug is freed, but then is too loose, use a piece of teflon plumbing tape on the slug thread, or a small piece of rubber band between the slug and former, to hold it in place. Or melt some wax down the hole. To do this, buy a candle and use your heat gun to control the melting.

If during alignment of the radio, when you turn the chassis over to access the slugs on the other side, and radio suddenly goes dead or insensitive, then a slug is loose, perhaps not connected to the adjustment screw. Dismantle the coil and slug and glue it back together. If the lock nuts on the rod trimmers are too loose, the rods will fall out when the chassis is turned over. When removing the case or turning a radio over, do not let it touch or damage the adjusting screws. Some radios can break off the tuning screws, when the chassis is inserted or removed from the case.

If a tuned circuit does not peak, then stop and find out why. Is the slug broken, or winding open circuit, or parallel capacitor the wrong value? Are you using the correct frequency? Are you turning the wrong adjustment? Is it loaded by something? Use your grid dip meter (GDO) and check the tuned circuit. An IF (Intermediate Frequency) transformer may have 2 positions where it tunes for a peak, are they the same, sometimes the one with the slug almost out, gives more gain. Most tuned circuits should be close to the correct frequency, and should only need a little re-tuning. If it requires a lot of change, find out why. Don t get confused during alignment and turn the wrong adjustment.

TYPES OF RADIOSThe most common radio is a superheterodyne. It has an IF amplifier, and may have an RF (Radio Frequency) amplifier. Most of the radio gain occurs in the IF amplifier, so it is important to get the IF aligned properly. The IF amplifier must be on the correct frequency, and don t assume it is 455 kcs, it may not be, as 175 kcs, 500 kcs, and 40 mcs are common IF frequencies. There are many others. The selectivity or bandwidth is effected in the IF amplifier, so all the tuned circuits, and any crystal filters or mechanical filters must all be aligned to the same frequency. The mixer contributes little or no gain, and is really just a frequency changer (sometimes it was called a frequency changer or first detector ). There may be more than one mixer, and more than one IF amplifier, which means that the radio uses double conversion (or maybe more). The RF amplifier adds some gain, but has other functions. Mixers are noisy, so any signal amplification before the mixer improves the signal to noise ratio. The RFamplifier also limits overload on strong signals, so that intermodulation products can be reduced, and effects on the mixer oscillator can be lessened. Intermodulation products, are the result of a nearby strong signal, which can drive the radio into overload, so that it acts like a mixer, and the strong station appears in many places on the dial. The RF amplifier also reduces the sensitivity to images. In the superheterodyne principle, the local oscillator ismixed with the desired receive signal, to make the IF signal for amplification.It also mixes equally well with any other signal, that is on the opposite frequency side of the local oscillator. A strong station may appear in two places on the dial. The RF amplifier reduces the second unwanted signal. To identify the correct signal, reduce the signal generator level, and the image (which is always weakest) will disappear first.

A crystal set, or TRF (Tuned Radio Frequency) radio, can be aligned the same way as an RF amplifier in a superheterodyne. A regenerative set, may be aligned the same way as an RF amplifier, provided the regeneration control is set so that it is not oscillating. A super-regenerative set, may be aligned the same way asan RF amplifier, provided the quench oscillator is disabled.

IF AMPLIFIER ALIGNMENTThe IF amplifier must be on the correct frequency, so that the RF circuits and oscillator will track correctly with the dial. Obtain the manual, or see if the IF frequency is marked on the chassis. A quick way is to feed a signal generator in, and that will give an indication. Be sure of the frequency before you start. Inject the signal at the IF frequency into the mixer grid, using a capacitor, so that any AVC or grid bias is not affected. Mixers should not have AVC, but some designs do. By injecting into the mixer grid, it is easy to align the first IF transformer primary, which is located in the mixer plate circuit. Reduce the input signal level, until you can just hear it above the noise. This will prevent overloading, make the tuning sharper, and remove any AVC effect. Keep changing the level during alignment. Peak all circuits for maxium output.

A common domestic radio, which has one IF amplifier valve, will have four tuned circuits in the two IF transformers. Three of them should peak nicely, but the last one is loaded by the detector and is very broad. You can try and disconnect the diode in the audio detector, but you will need a high impedance probe to measure the output level at this point. You won t achieve much improvement. Peak the four adjustments for maximum output. The IF alignment is finished.

Don t expect too much, as a domestic radio intended for city use, may only have a 1 mV sensitivity. War surplus communications receivers may have 5 uV sensitivity, and modern amateur receivers may have better than 1 uV sensitivity. Also note that the tuning coil on the mixer grid may load the signal generator. This may require more signal injection, if you change bands. It is not important.

A communications receiver, may have two or more IF amplifier valves. It may be necessary to disable the AVC during alignment, as this may mask the peaking. Some have a switch to tun the AVC off, when the BFO (Beat Frequency Oscillator) is turned ON. Otherwise, use a jumper wire from the AVC diode to earth. Make sure the RF gain control is turned fully ON. If there is a bandwidth control, then use the narrow setting, and check that the wider settings do not cause a gain reduction. A sweep generator is helpful here.

If there is a crystal filter, then check which type it is. If there is a Phasing control, then it is notch reject filter. Otherwise it is a normal narrow filter. The normal filter is easy, just align the IF to the crystal frequency. This may not be exactly the published frequency, however, this is the practical frequency to use, and it should not be far off. If the crystal doesn t work, dismantle it and clean with alcohol, or get another one. A sweep generator is also helpful here. If you don t have one, find the peak with the crystal switched in, and align on that frequency. Switching the crystal in and out, should change the gain little. If it is a notch type filter, align it with the crystal filter ON, but with the notch set to any side, away from the centre frequency. Check that after alignment, you can reject the signal generator when the control is centred, and also reject it, when the signal generator is slightly above and below the IF frequency, by using the phasing control. A sweep generator is very helpful here.

If there is mechanical filter, or a crystal filter module (this may have multiple crystals inside), then it will have a very steep skirt. Always align in the centre, as if you are too near the edge, there will be some phase distortion, which sounds muddy . A sweep generator is also helpful here.

Some communications receivers, especially the more modern wadley loop type, have broad IF filters, sometimes 1 mcs wide. The manual has instructions on how to align the filter, for a flat response, and skirt frequencies. The method can be time consuming, as the signal generator has to be set to several different frequencies, and different inductors have to be peaked and nulled, through an iterative process, with continual checking of the flatness. A sweep generator makes it easy.

BEAT FREQUENCY OSCILLATOR (BFO)After you have aligned the IF, set the BFO control knob to the centre of its range, and then align the BFO coil adjustment so that it is at zero beat, with the signal generator. You may wish to switch off the signal generator modulation while you do this. Then check that the beat note increases, either side of the centre position. You may also want to check that you can achieve a 1.5 kcs beat note on either side of the centre, so that sideband signals can be resolved. A frequency counter is useful for this. The BFO dial readings can be recorded.

MIXER OSCILLATOR ALIGNMENTThe mixer provides little or no gain, and is merely a frequency changer. It changes the received frequency to the IF frequency. So the attenuator settings for the IF amplifier should be about the same as the final IF alignment.

There are two main alignment goals, maximum gain (sensitivity), and dial accuracy. The dial accuracy must be done before you can do anything else. First adjust the dial pointer, so that as you tune from one end of the dial to the other, the pointer tracks correctly. Fix this before you go any further.

Then align the oscillator so that the received frequency matches the dial readings. This must be done at each end of the dial, so lots of tuning from one end of the dial to the other, will be done throughout the RF and oscillator alignment. It is handy to have a modern signal generator that can store two frequencies, and switch quickly between the two. The alignment must be done, not at the ends of the dial, but near them, about 10% in from each end. The manual will indicate the preferred frequencies for each band on a multi band receiver.

The alignment at each end of the dial affects each other, so always go back to the opposite end several times, until you are sure they are stable, they do not move, and are correct on the dial. The alignment is usually started at the low frequency end of the dial, as this has the greatest effect. Then align the high end. Then check the low end, and adjust. Then check the high end and adjust. Then the low end, then the high end, and perhaps it is correct. A ferrite slug is often the low frequency adjustment, and a capacitor trimmer, is often the high frequency adjustment.

The oscillator is aligned so that it is always tuning parallel to the received frequency, and the distance is always the IF frequency. It is usually above the received frequency, but do not assume this, check the manual. Also check that it stays on the same side, as it may swap to the lower side on a higher band.

Set the signal generator to the low frequency end, align the oscillator, change the signal generator to the high frequency end, align the oscillator, repeat until it is correct. Do a spot check in the centre of the band.

RF ALIGNMENTThis is the same routine. Set the low end, but now peak the alignment for maximum output. Set the high end and peak, then check the low end. Repeat till the maximum output is obtained. If it is a multiband radio, align the oscillator for each band, and the RF stages for each band.

AERIAL CIRCUIT ALIGNMENTThere is one special consideration. The aerial circuit is connected to your signal generator which has a 50 ohm output. This is fine if you receiver has a 50 ohm input. If it is an older receiver, it may be expecting a higher input impedance. If your receiver has an aerial trimmer control, then it does not require any further alignment, as the aerial can be adjusted at each frequency during reception. But other wise the aerial tuning will not be the best. One easy method, is to disconnect the aerial input, attach a short wire, and leave the signal generator cable close (but not touching) and peak the aerial coils. Another method, is to attach a real aerial, tune in a station, and align the aerial coils. The correct method, is to use your dummy aerial, connected between your signal generator and the receiver.

Individual Dummy Aerials

Combined Dummy Aerial

To further increase your receiver sensitivity and selectivity, correctly match the aerial to the receiver. Your receiver input impedance will change as you tune across the dial. Your aerial impedance will also change with frequency. Use an Antenna Tuning Unit (ATU) designed for transmitters, and match your aerial and receiver to each other. Tune it for maximum signal. When tuning around, have this ATU bypassed. When you have found the station you want, switch itin and adjust it. Some receivers have a preselector that has a similar function.

COMPLETIONPut a label or tag on the radio with the alignment date, and sensitivity. Seal theadjustments with wax. Reassemble the radio. Leave it running for a while, to verify that the alignment does not change.

TEST EQUIPMENTYou can use many pieces of test equipment, but a signal generator is the most useful.

SIGNAL GENERATORThe signal generator you use should cover the IF frequency, which may be 50kcs to 500kcs, and is often 455kcs. Do not assume it is, check the documentation. For alignment of the RF and mixer stage, you will need to cover the receiver frequency, which may be 500kcs to 30mcs. The signal generator requires an output attenuator, so that it can be turned down, as the alignment progresses. You need to go down to 1 microvolt or better. Beware that some signal generators may leak RF, so that the attenuation has little affect on the reduced levels. The signal generator needs some modulation facility, which is normally AM, and normally 100 cycles or 400 cycles, and normally is set at 30 percent. Some have external inputs for other frequencies. FM modulation may be required for higher frequency receivers.

LEVEL METERYou can align radios for the greatest audio output, but if you use a level meter on the output, you can get better accuracy and get more overall gain. The best meter is an Audio Output Meter, or RMS Level meter, or a Noise and Distortion Set (switched to Level), or even a multimeter switched to Output. Any meter will be suitable. Using a signal 10 dB above the noise level, is a good rule of thumb. Keep adjusting the signal generator level during the alignment.

OSCILLOSCOPEThis may be useful for finding any distortion in the circuitry, and identifying any overload conditions. It is also useful for locating any noise or hum that may be introduced and confuse the alignment. It is also required for use with a sweep generator.

SWEEP GENERATORAlso called a wobbulator. This is useful for showing the overall passband and allowing symmetrical alignment of the IF amplifier. It makes alignment quicker. Especially in complicated passband filter alignment. It is a little complicated to set up, but is very useful for crystal filters. Sweeping over the passband too fast will skew the shape, so slow the sweep speed down till it looks reasonable. Markers are required so that you can set the centre frequency or the side bands. If it does not have any markers, switch to manual, connect a frequency counter, set the centre frequency manually, adjust the horizontal shift of the oscilloscope so that it is on a graticule, then switch back to sweep. The graticule can be used as the IF centre frequency marker. Do not touch the horizontal shift or manual sweep again.

FREQUENCY COUNTERThis can be useful to check your signal generator or sweep generator.

GRID DIP OSCILLATOR (GDO)This can be useful to find out what the resonance of a tuned circuit is, especially when it will not align as expected.

NO EQUIPMENT AT ALLYou can align a radio without any equipment, if you are careful, and if you don t expect outstanding results. First assume the IF frequency is close enough to the correct frequency. Tune the radio between stations, so that there is only noise. Peak each IF for maximum noise. Tune in a known station on a known frequency at each end of the dial and align the oscillator. Tune between stations at each end of the dial, and peak for maximum noise. Always align the low frequency end first. Good luck.